The present invention relates to control of temperature by means of a CCD-camera when welding optical fibers, in particular optical fiber ribbons and devices for accomplishing such control, and it also relates to a method and a device adapted to weld fiber ribbons to each other.
A very important parameter when splicing optical fibers by welding is the temperature at the welding location. To be able to maintain the temperature of the fibers at a sufficiently high level and during a sufficient long time period are determining factors in order to obtain a low attenuation and a high mechanic strength in the splice produced.
A known method for an indirect control of the temperature of the fibers, which is used in some known welding devices, is called xe2x80x9cMeltbackxe2x80x9d, see the article by G. Kiss, xe2x80x9cHigh yield fusion splicing in the outside plant: using fiber meltback to monitor electrode conditionxe2x80x9d, National Fiber Operation Engineering Conference, Denver, USA, September 1996.
In Patent Abstracts of Japan, abstract of the Japanese patent application No. 2129607, a fusion splicing device is disclosed, in which the brightness of an optical fiber heated in an arc discharge is observed. The discharge heating temperature is checked from the area of the bright part of the optical fiber.
When optical fibers assembled to optical fiber ribbons are to be spliced by melting the ends of the optical fibers in an electric arc, a problem exists of making the ends of the opposite individual fibers contact each other before and/or in the very fusioning process. This is due to the fact that it is very difficult to cut off such fiber ribbons at an exactly straight angle in relation to the longitudinal direction of the respective fiber ribbon. Also, the operation of stripping the polymer protective coating of a fiber ribbon and the force then used can result in that some fibers in a fiber ribbon will be elongated more than other fibers. Hence, the end surfaces of the fiber ribbon will not even be located in a plane.
In Patent Abstracts of Japan, abstract of the Japanese patent application No. 5-142442, a fusion splicing method for xe2x80x9cmultiple fibersxe2x80x9d is disclosed, in which the connecting ends of optical fibers are preheated xe2x80x9cat a low temperature to the extent of not attaining an excess molten state. The molten end faces of the optical fibers are then pressed to each other and while the end faces are heated in the state at the temp. higher than the above-mentioned preheating temp., the optical fibers are pushed in, by which the optical fibers are fusion spliced.xe2x80x9d
It is an object of the invention to provide methods and devices for temperature control in welding optical fibers, which only use devices existing in conventional automatic welding machines, and in particular to provide methods for a simple determination of a suitable welding temperature.
It is another object of the invention to provide a method and a device which in a secure way can accomplish welded splices of fiber ribbons.
Thus the optical system and the CCD-camera, which is arranged in conventional automatic welding devices, are used for determining the light intensity in a picture of the optical fiber, when it is heated, a locked control of the CCD-camera being used, so that the automatic light intensity setting system thereof is shut off.
The light intensity in a picture captured of heated optical fibers is directly related to the fiber temperature according to Planck""s radiation law. It is used for an active control of the fiber temperature in the procedure and when in advance determining suitable welding currents.
The method makes it possible for the welding device and the user to compensate for the influence of some environmental factors such as under-atmospheric pressures and electrode condition, which often cause a lowering of the temperature during the welding process, when welding parameters are used which are programmed for normal situations.
When welding optical fibers to each other, which can be optical fibers assembled to optical fiber ribbons, the following steps are used:
a. The optical fibers are heated to a welding temperature by means of an electric arc between electrodes, between which an earlier determined electrode current passes. The electrode current has here preferably been determined to always give a not to high temperature suited for welding the fibers.
b. The intensity of light emitted by the heated optical fibers is determined and is compared to a predetermined set value.
c. Thereupon, in the case where the intensity of the emitted light has been determined to deviate from the desired value by more than a predetermined amount, the electrode current is changed, preferably increased by a predetermined step, whereby the temperature of the optical fibers is also changed by a corresponding step, and thereupon the steps b. and c. are again repeated, and in the case, where the intensity of the emitted light has been determined to deviate from the set value by less than the predetermined amount, the heating is allowed to continue at this electrode current, whereby the welding is made.
A similar procedure is as follows:
a. The optical fibers are first heated to a temperature, at which they emit visible light but which temperature is well below the temperature, at which the material of the fibers melt and at which the welding is to be made.
b. The intensity of light emitted by the heated optical fibers is determined and is compared to a predetermined set value.
c. Thereu{acute over (p)}on, in the case where the intensity of the emitted light has been determined to be less than the set value, the temperature of the optical fibers is increased by a predetermined step, and thereupon steps b. and c. are again performed, and in the case where the intensity of the emitted light has been determined to be greater than or equal to the set value or to deviate from the set value by less than some small, predetermined value, the welding is made at that temperature.
A corresponding method can preferably be used for in advance determining an electrode current, which gives a desired temperature of an optical fiber, e.g. the temperature to be used in welding the fiber to a similar fiber or a temperature to be used in a softening step. The optical fiber is as above heated in an electric arc, through which an electrode current passes. The following steps are then performed:
a. An electrode current having a first value is made to pass in the electric arc in order to heat the optical fiber to a temperature, at which it emits visible light, but which is well below the desired temperature.
b. The intensity of light emitted by the heated optical fiber is determined and is compared to a predetermined set value, which is determined depending on the desired temperature.
c. Thereupon, in the case where the intensity of the emitted light has been determined to be less than the set value or generally to deviate from the set value by not less than a small, predetermined value, the electrode current is increased by a predetermined step and thereupon steps b. and c. are again performed, and in the case where the intensity of die emitted light has been determined to be equal to or higher than the set value or generally to deviate from the set value by less than the small predetermined value respectively, the used electrode current is taken as the electrode current, which gives the desired temperature. If the desired temperature is the welding temperature, the determined electrode current can then be used for welding the fiber to another fiber, whereby a correct temperature during the welding process can be achieved. The predetermined step can e.g. be equal to the small predetermined value or a little smaller.
In the step b. of the procedures described above for determining the intensity of light emitted by a heated optical fiber this determination can be made by
capturing a picture of the heated optical fiber, and
analysing the picture to find values corresponding to the intensity of light only in regions of the pictures of the optical fiber corresponding to a portion of the optical fiber, which substantially corresponds to a central part of the optical fiber. The portion can have a width, as taken perpendicularly to a longitudinal direction of the optical fiber, which is substantially equal to half the diameter of the optical fiber. In the analysis the regions can be selected to be located at an end surface of the optical fiber. Further the picture can be analyzed only along lines which extend substantially perpendicularly to the longitudinal direction of the optical fiber to find the values corresponding to the light intensity. These lines are preferably located near the end surface of the optical fiber.
The procedures described above are, as has already been mentioned, particularly suited to splicing optical fiber ribbons. When splicing an end of a first optical fiber ribbon to an end of a second optical fiber ribbon by means of welding, ends of optical fibers in the first optical fiber ribbon are welded to opposite ends of optical fibers in the second optical fiber ribbon. Then the following steps can be employed:
Referring to FIGS. 7a-7e, FIG. 7a shows the first 700 and second 710 optical fiber ribbons are placed and aligned with each other, so that an end surface of an optical fiber in the first optical fiber ribbon 700 is located opposite and at an end surface of an optical fiber in the second optical fiber ribbon 710.
The arrows shown in FIG. 7b indicate that the end surfaces are moved in a direction to approach each other from a first position at a small distance of each other to a second position in which an end surface of at least one optical fiber in the first optical fiber ribbon 700 is in contact with an end surface of an optical fiber in the second optical fiber ribbon 710. Then the end surfaces are moved in the same direction, as indicated by the arrows of FIG. 7c, from the second position to a third position in which the end surfaces of all optical fibers in first optical fiber ribbon 700 are pressed against end surfaces of optical fibers in the second optical fiber 710. Then, as shown in FIG. 7d, opposite fiber ends will deform, mostly elastically, allowing this contact between all opposite fibers, this step being said to produce an overlap. The end surfaces are maintained in the third position during a first time period. The ends of the optical fibers in the first 700 and second 710 optical fiber ribbon are during the movement between the first and third positions maintained at a first temperature and during the first time period at a second temperature equal to or higher than the first temperature. The first and second temperatures are selected to be sufficiently high in order that the material of the ends of the optical fibers will soften, so that the ends of the optical fibers can be somewhat deformed in the pressing operation of the end surfaces against each other, but not being sufficiently high in order that a melting or fusing of the ends or a joining of the ends of opposite optical fibers in the first and second optical fiber ribbons will take place. The first period, during which the end surfaces are pressed against each other, is taken to be so long as to achieve such deforming of the ends of opposite optical fibers, that elastic stresses in all pairs of opposite end surfaces of the optical fibers are substantially relieved or relaxed. This is particularly important for the fiber pairs, the end surfaces of which at the beginning of the pressing operation have first contacted each other and which thus have been most deformed in the pressing operation between the second and third positions.
During a second time period subsequent to the first time period the heating of the ends of the optical fibers is continued at a third temperature higher than the first and second temperatures. The third temperature is selected to be so high that a melting and thereby a joining of opposite ends of the optical fibers takes place and the length of the second time period is selected accordingly to allow the melting and joining. Owing to the previous relaxing of elastic stresses and strains the ends will during this fusioning step be minimally deformed and in particular the cores of the fibers will maintain their straight configuration and a thus the loss in the splices formed will be at a minimum.
The velocity of the movement of the opposite end surfaces between the second and third positions can preferably be lower than the velocity of the movement between the first and second positions, e.g. be substantially equal to half the latter velocity.
During a third time period which can be selected to coincide with an end period of the second time period or which starts when the ends of the optical fibers have been maintained at the second temperature during a predetermined time period, and as indicated by the arrows in FIG. 7e, the ends of opposite optical fibers can be pulled away from each other through a predetermined, short distance in order to straighten the deformed fiber ends a little. This short distance can be a little smaller than the distance used for producing the overlap, e.g. be 0.5-0.9 thereof. The velocity of the movement between the second and third positions can then be substantially equal to velocity of the movement used when the end surfaces are pulled away from each other.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the methods, processes, instrumentalities and combinations particularly pointed out in the appended claims.